Biochemistry 600 Quiz 2

Indicate whether the following statement is true or false; Despite the presence of more than 300 amino acids, DNA codes for incorporation of only 20 D- amino acids in vertebrates

DNA codes for only 20 amino acids, but does it code for D- or L-amino acids. It codes for 20 L- amino acids NOT D-amino acids. Statement is false.

DNA contains which one of the following components? (A) Nitrogenous bases joined by phosphodiester bonds (B) Negatively charged phosphate groups in the interior of the molecule (C) Two strands that run in the same direction (D) Hydrophobic base pairs stacked along the central axis of the molecule (E) The sugar ribose

Response A: It is true DNA has nitrogenous bases and has phosphodiester bonds. It is, however important to note that the nitrogenous bases are linked to the sugar via N-glycosidic bonds and not through phosphodiester bonds. DNA Phosphodiester bonds link one sugar to the next. This statement is therefore not correct. B: DNA has negative;y charged phosphate groups and as a resultant DNA is negatively charged. The negative groups are, however, not found in the interior of DNA but rather point outwards. C: DNA has two strands that run in opposite directions. (Correct Answer) D. The hydrophobic base pairs of DNA are indeed stacked along the central axis of the molecule E: In DNA, the sugar is deoxyribose, ribose is the sugar in RNA.

Regarding weak interaction forces in biomolecules all of the statement(s) below is/are correct except (A) They include hydrogen bonds, hydrophobic interaction, Van der Waals, and covalent bonds (B) They play a significant role in stabilizing proteins and other macromolecules like DNA (C) They are important in the binding of substrates to enzymes (D) A and C (E) All of the above statements are correct

Response A: While hydrogen bonds, hydrophobic interaction, Van der Waals are examples of weak interaction forces, covalent bonds are not. Covalent bonds are the strongest bonds and there is no way they can be classified as weak interactions. Statement A is incorrect. Responses B and C are correct as weak interacting forces play an important role in stabilizing proteins and other macromolecules like DNA and are involved in binding of substrates to enzymes.

Pertaining to hydrogen bonds in water molecules, which of the following statement(s) is/are correct? (A) Water has a significantly high boiling and melting point because of hydrogen bonding. (B) Because of hydrogen bonding water increases in density as it freezes making ice float on water, which prevents oceans from freezing from the bottom. (C) The presence of hydroxyl groups in alcohol prevents its formation of hydrogen bonds with water molecules resultantly alcohol does not mix with water. (D) A and B (E) All of the three statements above are correct. The question is testing your knowledge of hydrogen bonds and how they influence water properties.

Statement A is correct. Water has a much higher boiling point and melting point compared with other solvents and closely related compounds largely because of the influence of hydrogen bonding between water molecules. Statement B: Due to hydrogen bonding acquired properties water decreases (not increase) in density as it freezes making ice float on liquid water which prevents oceans from freezing from bottom going upwards, the consequence of which would have been permanently frozen oceans and catastrophic to ocean life. Statement is wrong Statement C: One of the significant hydrogen bonds occur between hydroxyl groups of alcohols and water. Statement is therefore wrong.

Given that a collected bacterial DNA sample has 20% guanine, what is the proportion of adenine? (A) 20% (B) 30% (C) 40% (D) 60% (E) 80%

The question again seeks to implore your knowledge of base pairing. Based on Chargaff's rule, the proportions of guanine is always similar to cytosine and that of adenine to that of thymine for any given species irrespective of nutritional status, age, environment or tissue of collection. This implies that if guanine is 20%, cytosine is 20%. The remaining 60% is equally divided between adenine and thymine (60/2 = 30%). B is therefore the correct option.

Which of the following statements regarding RNA is correct? (A) Uracil is replaced by thymine (B) When base-pairing occurs in loops of RNA, adenine is hydrogen bonded to uracil (C) RNA is tightly bound to histones forming nucleosomes (D) B and C are correct (E) None of the above

The question as it states wants to test your understanding of RNA properties. Response A: It is true that unlike in DNA where there is thymine, in RNA, the thymine is replaced by uracil. Response A suggests that thymine is the one that is found in RNA replacing uracil. Statement is not true. (Correct) Response B: While RNA occurs largely in the single stranded form, whenever base-pairing occurs in loops of RNA, just like in DNA, a purine will always pair with a pyrimidine. G will pair with C and A will pair with Uracil. Statement is true. Response C: While DNA is tightly wound around histones forming nucleosomes, mRNA is usually processed as it is synthesized and is not stored in the cytosol. Logically, there is no need for histones to bind to RNA and package it. Statement is false.

6. Which of the following statements regarding cholesterol is not correct? (A) Cholesterol is a 27 carbon compound that has four ringed structures (B) Cholesterol is a precursor of steroid hormones (C) Cholesterol is a precursor of bile acids and bile salts (D) Cholesterol has a non-polar head that is hydrophilic (E) None of the above responses is incorrect

The question is accessing your knowledge of cholesterol. Cholesterol is a 27 Carbon compound with 4 ringed structures and is a precursor of steroid hormones, bile acids and bile salts. Because of the presence of a hydroxyl group the head of cholesterol is hydrophilic. Non-polar compounds are hydrophobic. Statement is, thus, false making D the correct answer.

Which of the following statement(s) regarding fatty acids is/are true? (A) Vertebrates lack the capacity to produce certain polyunsaturated fatty acids and must obtain them from the diet. (B) Unesterified fatty acids are normally transported in plasma non-covalently bound to proteins like albumin. (C) Physical properties of fatty acids depend on the length of the chain and the level of saturation. (D) A and C (E) All of the three statements are true.

The question is testing your knowledge of fatty acids/lipids in general. A: Vertebrates require to take essential polyunsaturated fatty acids in the diet because they lack capacity to insert double bonds beyond Carbon 9 (will be covered in more detail under fatty acid metabolism): Statement is true. B: Because free fatty acids (unesterified) are hydrophobic, they are transported bound to proteins like albumin. The binding to proteins is non-covalent. Statement is true C: It is true indeed that physical properties of fatty acids depend on chain length and level of saturation. The three statements are, thus, correct, which makes option E the most appropriate choice.

In DNA, on a molar basis (A) adenine equals uracil (B) adenine equals thymine (C) guanine equals adenine (D) cytosine equals thymine (E) thymine equals uracil

This question is testing whether you know the base pair combinations for DNA. A purine will always pair with a pyrimidine. Adenine pairs with thymine (A-T) and guanine pairs with cytosine (G-C). Uracil is only found in RNA. The correct answer therefore B.

Regarding sphingolipids, which of the following statements is false? (A) They have a sphingosine backbone (B) Glycosphingolipids contain two or more sugars (C) Sphingomyelinsareamphipathicandformanimportantcomponentofthemyelinsheathof neuronal cells (D) Most sphingolipids are derived from ceramide (E) None of the above

This question was meant to test your general knowledge of sphingolipids. Due to a technicality in wording of statement I ended up disregarding this question and giving points to everyone. It is correct that sphingolipids have a sphingosine backbone (A) and most of them are derived from ceramide (D). A class of glycosphingolipids known as globosides contains 2 or more sugars, while the other class called cerebrosides may contain one sugar. Statement C is correct. By omitting the word may contain 2 or more sugars in the wording of statement 2, the interpretation was justifiably different and I had to nullify this question and award the point to everyone.